In Chapter 1002, when Zoro was preparing Hiryu Kaen we see something gather around his sword.
There seems to be no consensus on what exactly the aura we see building up around Zoro’s sword is. Below, I would try to perform a comprehensive analysis of what the mysterious aura might be, based on what we’ve been shown in the manga so far.
I think it’s very unlikely that Hiryu Kaen is just fire. It is shaded far too darkly to be a sword on fire.
This is how Oda draws Zoro’s swords on fire:
This is how Oda draws the fire Kinemon’s Foxfire technique produces:
This is how Oda draws the flames generated by Hiryu Kaen:
In contrast the aura around Zoro’s Enma is much darker:
Giving the dark near black shading, I think it’s very unlikely the aura around Enma is mere fire.
I think there are reasons to believe that Haki might be the source of the aura around Hiryu Kaen.
- Before preparing the attack, we see an aura gather around Enma that is clearly not fire:
- The mysterious aura seems like Ryuo to me.
- Enma is hardened, so Zoro is clearly imbuing it with Haki:
- Enma’s special property is to force its wielder to release excessive amounts of Ryuo:
- Given what we know of Enma’s ability, a mysterious aura building up in large amounts around Enma before a powerful attack is probably Ryuo.
- Release of large amounts of Ryuo is how Enma produces large slashes.
- Hiryu Kaen did in fact produce a very large slash:
- It’s normally a close range attack and not a flying slash, so the excessive range was probably produced by the release of Ryuo.
- Haki/Ryuo is the main auxiliary power source in One Piece, and the source of most auras, so a safe bet is that this unknown aura is probably Haki.
All the above said, there are reasons to be sceptical that the aura that built up around Zoro’s Enma was Haki alone. It is drawn pretty differently from other instances of Haki we’ve seen.
When Haki overflowed from Zoro’s swords when he blocked the Bird Cage, it was pitch black:
When Haki overflowed from Zoro’s swords during Rengoku Onigiri, it was pitch black:
When Zoro first used Enma, the Haki that was overflowing from his sword was pitch black:
When Haki overflowed from Whitebeard and Roger’s weapons during their clash, it was also pitch black:
Given that all previous uses of overflowing Haki were pitch black, I think it’s unlikely that this is a usage of just overflowing Haki and it happens to not be pitch black.
Based on all the information available, I think the aura that built up around Zoro’s Enma was fire that had been imbued with overflowing Haki (hence the near black colour). If you look closely, the shape of the flames resembles the shape of the flames produced by Kinemon’s Foxfire style (how Zoro might have generated such flames in the first place):
I don’t think that’s just a coincidence, and we know that Zoro was using Haki when he prepared the attack. Zoro’s overflowing Haki infused with the Foxfire flames and turned them black.
*Theory by Cinera